i have just booked an I class fare LIN-LHR-BKK-LHR-LIN for later this month - originally I wanted to stopover in LHR (ie at home!) for around 30 hours - could not do on-line as you say, so phoned and was told any stopover more than 24 hours would add Euro 200 to the price.

So am doing it in one go, using a cheapo one way BMI ticket to get out there. Still, worth it for a CW return to BKK for £1270 ...and I get the 170 points I needed before July 8th for Silver.